1. Field of the Invention
The present invention relates to electrical power distribution systems and more particularly to filters for mitigating harmonics currents in such systems.
2. Description of the Related Art
Electrical distribution systems, for example in a building or industrial facility, supply electricity to non-linear loads, such as adjustable speed motor drives, uninterruptible power supplies (UPS), battery chargers, servo drives, and welders. These loads produce various levels of current at harmonic frequencies in the power distribution system with the harmonic spectrum depending upon the nature of the load.
the harmonic currents create many problems in the power distribution system, including increased voltage total harmonic distortion level, reduced power distribution equipment reliability, reduced electromagnetic compatibility of the loads, increased power losses, reduced power factor, and other problems which are well known to those skilled in the art.
Many types of apparatus have been devised for mitigating harmonic currents. Such apparatus include power factor corrected power supplies, active filters which inject harmonic currents having a polarity opposite to that generated by the load, resonant L-C filters, AC chokes, and phase shifting systems. As is well known, power factor corrected power supplies are expensive and have a limited application; active filters have a high cost, poor reliability and poor dynamic characteristics and may not be practical with certain loads; resonant L-C filters have a high cost, poor effectiveness in low voltage systems and the tendency to cause the system to operate with a leading power factor; AC chokes have a high voltage drop across the reactors; and phase shifting systems are typically limited in the number of harmonic orders which can be mitigated and the degree of harmonic mitigation depends upon the extent to which harmonics produced by the various harmonic sources are identical and their phase shift angles.